The detection of electromagnetic radiation in the visible and near visible regions of the electromagnetic spectrum has stimulated scientific and technological interest for many years. Interest in such detection was perhaps first stimulated by the desire to detect light from distant stars and as a result telescopes were developed for optical astronomy.
For many purposes, methods of detection other than the visual methods first used by astronomers are desirable if not mandatory. The methods that have been developed are varied and include photographic emulsions and solid state devices that use photoconductors, p-n junctions, charge coupled devices, etc. These methods have been developed to their highest technological levels for different specialized uses. For example, photoconductors are often used to measure light intensity for photographic purposes and highly sensitive photodiodes have been developed for use in optical communications systems using glass fibers.
The interest in photodetectors for specialized purposes has often been stimulated by developments in other areas of technology. For example, the development of tunable lasers emitting very brief light pulses, nanoseconds or shorter, that are typically of very high intensity has for the first time opened many new opportunities for time resolved spectroscopy of very fast physical, chemical and biological phenomena. Unfortunately, the detection of such pulses has proven to be more difficult than their generation.
Several detection systems are currently available and used to study very fast phenomena. Streak cameras presently have the best time resolution but suffer the significant drawbacks of being both complex and expensive. For example, a price of $50,000 per camera is typical. Semiconductor junction diodes are often employed for the alignment and monitoring of pulses from picosecond lasers. However, although a response time of approximately 50 picoseconds has been reported in the literature, the commercially available detectors generally have response times of only 100 picoseconds. The response time of this and the other available optical detection systems is slower than the optical pulses and complicated nonlinear optical correlation techniques must be used to obtain the desired time precision. Faster response times would still be desirable.
One field that has stimulated considerable excitement in recent years, especially the last 20 years, is that of amorphous semiconductors. The materials have attracted attention because of their photosensitive properties. This field was first of interest because of the well known xerographic copying process that uses arsenic doped amorphous selenium. More recently, memory devices and solar cells using amorphous semiconductors have generated widespread interest although difficulties have been encountered in reproducing results.
The experimental work has been accompanied by an increased theoretical understanding of amorphous semiconductors. The understanding has led to an appreciation of the importance of localized states such as defects. A complete understanding of the effects of localized states and defects has not, however, been achieved. Nor has a complete understanding of the effect of impurities, e.g., hydrogen in amorphous silicon, been achieved.
Due to the lack of theoretical understanding, as well as other reasons, amorphous semiconductors have not been successfully used as high-speed photodetectors, i.e., detectors having response times less than 50 picoseconds.